This CAN'T be good - piston damage

[Moody Blue]

This is a parts motor I acquired for the tilt/trim system. I did a compression test and found 155-165 PSI on #1,2,4 and 0 PSI on #3. Tore the motor apart and found this (see photos).

What would cause such destruction? In one of the photos you can see the inside of two of the cyls. You can see the shrapnel embedded in #3 cyl. The crown (proper term?) of every piston in this motor has some degree of erosion (see photo). What causes this? I have pistons from another identical motor that does not show such erosion.

What I want to do is rebuild "a" motor. I have a couple basic questions that will determine whether it is worth the effort or not. I have three complete identical motors, one is a runner on the boat, the other is in pieces in a box and the third is in process of being torn down. So I have two parts motors.

1) Can I interchange piston pins from one piston to another ?
2) Can I interchange connecting rods from one crank to another?
3) Is it acceptable to install new rings then install into existing honed cylinders?

The good parts from each motor are in remarkably good condition. Unfortunately there are also defective parts on each motor. I would need to combine parts from both parts motors to make one. My hope is that I can pick and choose all of the good parts, put them together and make one good motor.

 

[grootski]

Re: This CAN'T be good - piston damage

What engines are you working on?

 

[lowkee]

Re: This CAN'T be good - piston damage

I'd say unless you want to chase engine problems forever, invest in new pistons, new rings, new rod bearings (the rods are okay to reuse) and bring the block, rods and crankshaft to a machine shop to prepare for a rebuild. If you are wondering why you need new pistons, it's because once you hone cylinders, they are now bigger than the piston and you need a piston to match the new diameter.

It will cost you about $900-1,100 total if you shop around. I say 'invest' in this stuff, because you will have a nearly new engine after all of this (which will run *and last* like new. You can either sell it for a good chunk or replace your existing engine and sell the current one.

 

[sschefer]

Re: This CAN'T be good - piston damage

The damage you have could be from a cylinder that was running very lean. Could have been from a bad carb. Since it's a parts motor you probably don't know how it was running just before it seized. My guess is that it overheated.

 

[sschefer]

Re: This CAN'T be good - piston damage

My guess it that the cylinder was running lean and the engine overheated.

 

[Laddies]

Re: This CAN'T be good - piston damage

I believe sshefer is correct about the dirty carb both the botton 2 pistons show detonation. As far a mixing parts with the exception of the wristpins it don't hurt a thing as long as you use new bearings and rings and the replacements are within specs. Before honing the bad cyl. wash the burned aluminium out of it with muratic acid before honing.

 

[Moody Blue]

Re: This CAN'T be good - piston damage

Sorry forgot to mention one key detail. The motor is a 1971 4cyl Merc 800.

Thanks for your thoughts. I've done alot of repair work to different motors but never a rebuild. Why the need for new bearings? The bearings and bearing surfaces on the rods, crank and wristpin look flawless, truly. Even the cages appear as new.

Laddies, you mention "with the exception of the wristpins". Why are they not interchangeable? Here is a summary...

Motor #1 - bad crank, bad rods & bearings, good block, good pistons
Motor #2 - good crank, good rods & bearings, bad block (I think), bad pistons.

I was hoping to take the good pistons from motor #1 and marry them to the wristpin/rod assy's from motor #2. I then would like to reinstall these onto the same crank they came off of (motor #2) and put that "new" assy into the block of motor #1.

The block with the shrapnel damage in the spark plug area (photo in first post) is questionable to me. The cyl wall looks OK other than some aluminum deposits. Would I be OK to rework this block? I can try to get better photos of the damage if that would help.

One last question, does honing actually increase the dia of the cyl beyond the point that stock rings would be too small? I don't want to bore the cyls, just hone them. Again, never done this before so learning as I go.

Thanks again for your help.

 

[Laddies]

Re: This CAN'T be good - piston damage

I won't reuse needle brgs because it is impossible to know in which way they turned the first time and it's easier and safer as I must stand behind each engine I rebuilt. On the other hand I know several guys that mic each brg and reuse them in their racing engines. As far as switching wrist pins and pistons they a fit to one another but if you take the time to refit them it would work. As far as the damage to the combustion chamber just use a long punch with a flat end and peen down the sharp places and it should be OK. As far as the bore do not hone any more than necessary and check with a feller gauge for clearance.

 

[Faztbullet]

Re: This CAN'T be good - piston damage

Looking at your pictures the damage is typical trigger failure. The old distributer type ignition will cause preignition on #1 & #3. The other pistons look eroded also.Do not reuse this trigger!!(it likely the horseshoe style) And you can bet the cylinders are out of round and that block will have to be bored. I think I still have a old 10 bolt 80hp block if you need one, I have a complete fresh 8 bolt ready also

 

[sschefer]

Re: This CAN'T be good - piston damage

Looking at your pictures the damage is typical trigger failure. The old distributer type ignition will cause preignition on #1 & #3. The other pistons look eroded also.Do not reuse this trigger!!(it likely the horseshoe style) And you can bet the cylinders are out of round and that block will have to be bored. I think I still have a old 10 bolt 80hp block if you need one, I have a complete fresh 8 bolt ready also

I completely agree, after reading Faztbullets post and then going back to the pic's, a bad trigger is the most likely culprit on this engine. You could sort the bad from the good and see what you come up with. That will incurr addtional cost because unless you have a machine shop you can't accurately determine what is bad and what is not.

To answer the question of honning: Honing is done mearly to break the glaze on the cylinder walls or provide a seating surface after boring. Normally we buy .005 over rings and file the ends to achieve a proper end gap. You do that by placing the ring squarely in the cylinder and then measuring the end gap. File the ends as needed to achieve a precise fit.

Looking at your pics, my feeling is that you would have to take off too much material and using a hone would destroy the taper (if there is any from the factory) and do nothing for restoring the roundness. Accurate boring would be the correct method and that's a job for a machine shop.

As for bearings, replace them. With that kind of detonation they are probably pretty well hammered as well as the wrist pins. The rods might be O.K. but you need to take them to a machine shop and have them checked for straightness and they should be balanced.

It's my personal opinion (we are all entitled) that balancing the crank, flywheel, rods and pistons is critical. Outboard motors are precision built engines that are capable of sustaining high RPMS for extended periods of time. You may get away with slop on an old 350 chevy but you won't for long on a two cycle high rpm outboard.

Have fun, take your time and you might just end up with a great backup motor! If not, you'll definitely learn a lot.

 

[Moody Blue]

Re: This CAN'T be good - piston damage

SO there seems to be consensus. Now to determine the next step. I have measured (repeatedly and as accurately as possible) the cyl bores and pistons that I want to use. Here are the results.

Measured diameters 1/2" from bottom and top of each cyl. Measured along the crank axis and at 90* to the crank axis at both depths. All measurements were between 2.872 and 2.875. Factory spec is 2.872. My manual states that cyl hone finish is 2.875. This block (#1motor) had been in storage since 1987 after the previous owner started to tear it down but never finished the job. I took it apart and found what I think was water intrusion thru the bottom crank seal. The lower crank, #3 #4 rods and lower crank bearing were shot. It really has not had alot of hours on it. The cyls fortunately showed no signs of corrosion. I think this is the better of the two blocks.

Measured (engine #1) piston diameters above the rings and at the skirt. Measured along the wristpin axis and at 90* to the wristpin in both places. Diameters along axis of wristpin were 2.855 (spec 2.857) above the rings in both orientations. Diameters at the skirt were 2.863 along the wristpin axis and 2.855 at 90* to the wristpin (spec 2.865). Is this typical wear and tear? Are the pistons usable? They look in very good shape (no corrosion).

How can I check the rods for straightness?

As for oversized rings, I have only found 0.015 and 0.030 over. Do you think (from the measurements) that I could simply but stock rings and carefully hone the cyls? Seems like a shame to have to bore the cyls when they are sooo close to spec.

I will do as suggested and use all new bearings. Do you have specs on the wristpins? Can't find any.

I'm not clear on the trigger issue. Not really sure what the trigger is or where its located. There is a thin metal disk inside the distrib housing with 4 holes in it. The disk I think spins with the shaft and passes thru some kind of sensor. Is this the trigger?

 

[Laddies]

Re: This CAN'T be good - piston damage

You can not put oversized rings in a standard bore outboard. If you saying the cyls have a .003 out of round thats near max. but should run in a non piston ported engine, You should be able to help it with a AMMCO precision hone if you have one. Check the rods for straightness by laying them on a lapping block or piece of plate glass and then check centerline length. The piston clearance is loose for a good idle but should run. When parts don't meet specs. you are alway taking a chance on performance and reliability, back when we were in the rental business at a marina I worked at we have put the 44 cu.in. 50HP engines together looser than you engine is with no problems but I find it hard to tell you to try it as it's not my money and time involved. As far as the trigger problem I guess I never have heard of it and you showed only 3&4 pistons so I couldn't see any preignition on number 1, might be cause I not trying to sell things, that's against forum rules. Heres a spec chart for the engines.

 

[Moody Blue]

Re: This CAN'T be good - piston damage

Laddies, thanks for the chart. Will come in handy.

In fact, all the piston domes from that engine have moderate to severe corrosion of the piston dome. I have done some reading and came across info regarding the octane rating of the gas vs high compression motors. When I did a compression test on this motor, I was getting readings of 155-160PSI on the good cyls. Could this damage have been done by using low octane gas?

In your opinion should I invest the money and have the block bored? Roughly how much $$ to get that done? Would I just put in oversized rings or pistons and rings? I'd hate to end up with a motor that didn't run right when I was done but can't see spending lots of bucks on such an old motor either.

Still not clear on the wristpin issue. Can I install a different piston onto another rod/wristpin?

 

[Faztbullet]

Re: This CAN'T be good - piston damage

As Laddies posted you need to have a good hone run thru the cylinders (I use a sunnen 4 stone). 99% of the time it will leave a untouched area by the intake transfer ports and this will cause idle and off idle problems. As for low vs high octane fuel, the high dome engines runs great on 87 octane, just up high speeds .002 for additional cooling and reduce timing 2 degrees. Just bite the bullet and do it right. (bore and new pistons). What year is you other block as exhaust and mounting stud pattern is different between 1969-72 blocks and 73 and later.

 

[Moody Blue]

Re: This CAN'T be good - piston damage

Faztbullet, thanks for the info.
Will I need pistons AND rings AND rods with a rebore?

Can I just buy oversized rings and reuse existing pistons, rods and wristpins in the rebored cyls? I'm really delving into unknown areas here having never done this before.

There are things that SHOULD be done vs things that MUST be done. I am leaning towards the "should be done" approach, but not sure what that would involve. I will replace all the bearings and gaskets, that is a given.

Do you have any idea on the cost of reboring? Where would I look to have that done?

If I choose to rebore, how can I determine the best block to use?

Oh, all three of the motors are 1970-1971 vintage.

 

[Faztbullet]

Re: This CAN'T be good - piston damage

There is no such thing as oversized rings to fit a standard piston. When you bore oversize you must install a oversize piston, and the NEW pistons come with matching rings and wrist pins.You want to use the best block you have or the one with no damage in cylinder dome. Take block to a marine machine shop as these are a "blind" hole and most automotive shops cannot bore. Prices vary from city to city but I charge 45.00 per hole and that includes port chamfering, honed to finished size, bleed restricter modification, pressure wash block and oil down cylinders. You are also going to need a special set of ring compressers or have a dealer drop crank and rods in block for you. If you use Wiseco pistons have a .001 added to finish bore so piston dont stick. I do not use Wiseco in a inline unless I go .040 oversize.
You will need the following:
4- Pro 9015 .015 pistons
(dont order untill block is bored as you may have to go bigger)
4- Pro 596 rod bearings with cage
4- Pro 4036-1 wrist pin bearings
1- Pro 206 bottom bearing
1- Pro 206 w Upper bearing
1- Pro 4037-56 center main bearings
1- Pro 795 gasket set
1- Pro 2016-3 Upper/lower crank seal kit
2- Pro 7021 carb kits
1- Mercury 20140 distributer belt
All part #'s are Promarine numbers.

 

[sschefer]

Re: This CAN'T be good - piston damage

Just a quick note - I've since discovered that manually fitting rings is something that isn't normally done on outboards. As stated several times, you choice is std, .015 or .030..

Here's a link to give you an idea of what you get in the kit: http://www.sterndrive.info/mercury-outboard-parts/pistons_kits.html

Faztbullet, do you know if this if because they're indexed? That would make the most sense to me.

 

[Moody Blue]

Re: This CAN'T be good - piston damage

Faztbullet thanks for all the legwork getting those P/N's and for the clarification on the oversized pistons and rings.

Time for me to get some prices from local shops on machining the block.

 

[daveswaves]

Re: This CAN'T be good - piston damage

FAstbullet, good info and good to see someone still able to blind bore. I play with the inline 6's and rebuild them for a hobby, not commercially, just for the challenge and enjoyment of it. I run one on my bass boat as a daily runner.....can't beat em. The only thing that I would suggest to add to your input would be to set the timing to 21 degrees and make sure the distributor advance is all in exactly asthe carbs crack open. There is an inline filter built into the carb fuel bowl top that should be removed as well, merc had a bulletin on it but I can.t remember the number off hand. Thats likely what caused the dammage to begin with, carb leaning out as has been mentioned. These motors do not like to be lugged and if you are propped to run at 5000 rpm you are lugging and you will get dome damage as shown. The piston erosion is typical, for an engine that has been lugged and lean. Don't suppose you would like to share that bleed hole modification?

There is no such thing as oversized rings to fit a standard piston. When you bore oversize you must install a oversize piston, and the NEW pistons come with matching rings and wrist pins.You want to use the best block you have or the one with no damage in cylinder dome. Take block to a marine machine shop as these are a "blind" hole and most automotive shops cannot bore. Prices vary from city to city but I charge 45.00 per hole and that includes port chamfering, honed to finished size, bleed restricter modification, pressure wash block and oil down cylinders. You are also going to need a special set of ring compressers or have a dealer drop crank and rods in block for you. If you use Wiseco pistons have a .001 added to finish bore so piston dont stick. I do not use Wiseco in a inline unless I go .040 oversize.
You will need the following:
4- Pro 9015 .015 pistons
(dont order untill block is bored as you may have to go bigger)
4- Pro 596 rod bearings with cage
4- Pro 4036-1 wrist pin bearings
1- Pro 206 bottom bearing
1- Pro 206 w Upper bearing
1- Pro 4037-56 center main bearings
1- Pro 795 gasket set
1- Pro 2016-3 Upper/lower crank seal kit
2- Pro 7021 carb kits
1- Mercury 20140 distributer belt
All part #'s are Promarine numbers.

 

[Faztbullet]

Re: This CAN'T be good - piston damage

I will post pic's and tools needed. It take about 5 minutes to do .And you dont have to run the #99186 restrictors. I forgot about the inlets, I throew these away and install the flat top covers on my rebuilds and install the small white inline fuel filer off a V-6 to the fuel inlet. I think catching crap before fuel pumps is better idea than after fuel pumps. I got a super nice and clean 850 power port block STD bore if you need one. Sold all my I-6 blocks including my 150XS and J block. I kept a complete 77 115 for parts to keep my houseboat going.